Characterisation of gallium(III)-acetate complexes in aqueous solution:: A potentiometric, EXAFS, IR and molecular orbital modelling study

The aqueous gallium(III)-acetate system was studied in 0.6 M Na(Cl) at 25 degreesC using a multi-technique approach, including potentiometry, IR spectroscopy, EXAFS spectroscopy and molecular orbital calculations. The potentiometric data were satisfactorily explained by a model which includes one mononuclear and one binuclear complex. The corresponding equilibrium constants defined according to the reactions Ga3+ + HAc reversible arrow GaAc2+ + H+ 2Ga(3+) + HAc + 2H(2)O reversible arrow Ga-2(OH)(2)Ac3+ + 3H(+) are log beta(-1,1,1) = -2.08 +/- 0.09 and log beta(-3,2,1) = -5.65 +/- 0.06, respectively. The latter complex has a stoichiometry identical to that previously identified in the aluminium(III)-acetate system. Also in agreement with this system, the IR and EXAFS data strongly indicates that this complex consists of two edge-sharing Ga octahedra bridged with an acetate ion, and should be formulated [Ga-2 (mu-OH)(2) (mu-O2CCH3)](3+). This interpretation is further supported by the molcular orbital calculations.